In secondary batteries, which have recently been increasingly used, a lithium-containing cobalt oxide (LiCoO2) as a cathode active material is mainly used and, in addition, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure and the like, and a lithium-containing nickel oxide (LiNiO2) is considered.
LiCoO2 among the cathode active materials has excellent physical properties such as excellent cycle characteristics and thereby are broadly used. However, LiCoO2 is relatively expensive and charge/discharge current capacity thereof is low, approximately 150 mAh/g. In addition, a crystal structure of LiCoO2 is unstable at 4.3 V or more and thereby possesses a variety problems such as ignition due to reaction with an electrolyte.
Regarding this, technology for coating an outer surface of LiCoO2 with a metal (aluminum or the like), technology for heat-treating LiCoO2 or mixing LiCoO2 with other materials, and the like, such that LiCoO2 can operate at high voltage, have been suggested. Secondary batteries composed of such a cathode material are unstable at high voltage or are difficult to use in a manufacturing process.
Lithium manganese oxides, such as LiMnO2, LiMn2O4, and the like, are advantageous in that they contain Mn that is abundant as a raw material and environmentally friendly and thus are drawing much attention as a cathode active material that can replace LiCoO2. However, such lithium manganese oxides have low capacity and poor cycle properties.
Lithium nickel-based oxides such as LiNiO2 and the like are less expensive than cobalt-based oxides and, when charged to 4.3 V, the lithium nickel-based oxides have high discharge capacity. Thus, reversible capacity of doped LiNiO2 approximates to 200 mAh/g, which exceeds the capacity of LiCoO2 (approximately 165 mAh/g). However, LiNiO2-based oxides exhibit problems such as rapid phase transition of a crystal structure according to volumetric change through repeated charge/discharge, generation of a large amount of gas during cycling, and the like.
To address these problems, lithium transition metal oxides, in which some nickel is substituted with other transition metals such as manganese, cobalt and the like, were suggested. Although the nickel-based lithium transition metal oxide substituted with the metals has advantages such as relatively excellent cycle characteristics and capacity characteristics, there are still unresolved problems such as dramatic deterioration in cycle characteristics after extended use and stability problems during high-temperature storage.
In addition, mobile devices have been continuously reduced in weight and miniaturized, and, at the same time, are being highly functionalized by providing a variety of functions. In addition, secondary batteries attract attention as a power source of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like proposed as a solution to address air pollution due to existing gasoline vehicles, diesel vehicles and the like. Accordingly, increase in use of secondary batteries is anticipated and thereby the above problems as well as problems regarding a great quantity, battery stability at a high potential and high-temperature storage characteristics are being stood out.
Therefore, there is an urgent need to develop technology which is suitable for high capacity secondary batteries and may solve high temperature stability problems.